The present invention relates to laying underwater pipelines and, more particularly, to a system and method of concurrently trenching, laying and burying underwater pipeline.
Underwater pipelines often must be buried in the sea floor in order to protect them from icebergs, anchors, nets, currents, thermal instability and the like. The present state of the art is to either dig or plow a trench before laying the pipe, lay the pipe and then back fill the trench. Alternatively, the pipe is laid first and then either using jets, mechanical cutting machines or plows, the sea floor is cut underneath the pipe and allowing the pipe to settle in the trench. Both methods are very expensive.
Dredging or plowing in sand or loose material which is frequently experienced on continental shelves often results in slope sluffing and very high dredge quantities. Mechanical dredging from the surface is uneconomical over 100 meters and requires dredging from the seabed which means very low production and high costs. Plowing from the seabed has resulted in many bad experiences with many shut downs, high maintenance and therefore high cost.
Burying the pipe after it is laid is also very expensive because of the low production equipment involved. The large jetting barges and jetting machines require several passes because they have to work around the pipe. Both methods involve two different operations. To date, there are no successful systems or methods for concurrently trenching, laying rigid steel pipe and burying it all in one operation for large scale pipe (8″ to 72″) from either J lay or S Lay pipe laying ships.
As can be seen, there is a need for a system/method for concurrently trenching and burying while laying the underwater pipe. The present invention utilizes the resources of the pipe laying ship to create a high pressure seawater jetting system that trenches the pipe without slowing the pipe laying speed of the ship.